1. Field of the Invention
This invention relates to the field of devices that sense the presence of an undesirable fire or explosion within a protected area or compartment, and thereafter cause a fire suppressant to be released to extinguish the fire.
More particularly, this invention relates to a device for distinguishing a fire from the flash produced, for example, by a projectile penetrating a wall of the protected area, and for triggering the release of suppressant only when it senses a fire.
2. Description of the Prior Art
There are many situations where the protection of human life requires that an area or a compartment be protected from fires. For instance, the crew and passenger compartments and the engines of aircraft are areas where a fire can quickly cause disaster. However, the fire suppressant carried on aircraft adds weight that reduces performance, and generally only the amount of suppressant necessary to extinguish expected fires is carried. The timing of the release of the suppressant is critical. If released too soon, it may be exhausted before it is really needed; if released too late, it may not be adequate to suppress the fire.
Military vehicles, such as aircraft, tanks and personnel carriers, may be vulnerable to fires caused by the entry of projectiles or flak. When a projectile or a piece of flak pierces a wall of a compartment, it causes a flash of radiant energy in the ultraviolet, the visible, and the infrared spectral regions. Prior art fire sensors, depending on their individual capabilities, are susceptible to two problems--the fire sensor might interpret the flash as a fire and release the suppressant before the fire actualy developed; or even if the fire sensor determined that the flash was not a fire, it might interpret a quickly developing fire as the continued presence of the flash, and thereby fail to release the suppressant. (In technical literature, the words "detector" and "sensor" are sometimes used synonymously. Here, "detector" refers to a radiation sensitive element which converts electromagnetic radiation to electrical signals. The word "sensor" refers to a system using at least one "detector", and which includes some other electronic apparatus to amplify or process the "detector" signals.)
For example, the fire sensor system disclosed in U.S. Pat. No. 4,206,454 to Shapira, et al. is a system for sensing fires, but which is subject to reacting to suppress the flash caused by a projectile penetration. Shapira's sensor system has two sensor channels, a long-wavelength sensitive channel, and a short-wavelength sensitive channel. A projectile flash radiates a quick-rising short-wavelength component and a slower-rising long-wavelength component which can cause Shapira's device to activate the release of suppressant as soon as the long-wavelength component passes the threshold level of the long-wavelength sensitive channel. But, such operation might be unnecessary if no fire results from the projectile penetration, or might occur too soon if the fire ignition is delayed, as when leaking fuel is subsequently ignited.
The fire sensor system disclosed in U.S. Pat. No. 4,220,857 to Bright is likewise disadvantageous since it is also subject to interpreting a projectile flash to be a fire. On impact, a projectile often releases a small amount of incendiary material or produces an explosion, both of which generate a large amount of carbon dioxide as the product of combustion, even though the combustion may be very short-lived and produces no sustained hydrocarbon fire. Since Bright's system is intended to respond to an event wherein non-Planckian carbon dioxide molecule emission at 4.4 micrometers (for example) exceeds Planckian emission at adjacent wavelengths, the Bright system is subject to producing a fire output signal in response to such a flash. Thus, suppressant might be released to suppress a flash and explosion that would dissipate shortly by itself.
The fire sensor system disclosed in U.S. Pat. No. 4,101,767 to Lennington, et al. is also limited in its capability to distinguish a flash from a fire. The Lennington system is basically a single channel fire sensor (including a 4.4 micrometer detector 30) with a discrimination circuit (detectors 10 and 20) added to inhibit an output signal as long as the color temperature is greater than some value (e.g. -2400.degree. K.). This sensor system is explicitly adapted for responding to a HEAT round attack against an armored vehicle. In a HEAT round explosion, the 4.4 micrometer radiation component drops below the sensor threshold following the HEAT round impact before the short wavelength detectors signal a detected color temperature less than the preset value. In aircraft applications, however, projectile penetration flash radiation characteristics often follow a different pattern, and the Lennington system is susceptible to reacting to release suppressant to snuff such a flash that would actually dissipate rapidly by itself.